The overall objective of this research is to determine the effects of ischemia on the cell membrane and cell organelles in the pathogenesis of irreversible cellular changes and on the prevention of these alterations by drug therapy. The structural integrity of the cell membrane was investigated with extracellular tracers and freeze-fracture electron microscopy. While no changes were seen in the thin sections, the nicks in the lipid bilayer of sarcolemma began to appear after 45 min ischemia. The changes in the permeability varied from cell to cell after 50 min ischemia. Some cells were leaky and allowed lanthanum into their cytoplasm and others were unchanged. The gap junctions were also altered. Interparticle distance was increased significantly in in the ichemic cells and zones free of P particles were seen within the gap junction pathological changes were more obvious in the reperfused ischemic myocardium. The mitochondria were swollen and developed electron dense deposits which are known to be associated with irreversible cell injury. In situ chemical analysis of these mitochondrial deposits revealed presence of calcium, magnesium, and inorganic phosphorus. More interesting findings were obtained on the unfixed, frozen mitochondria which contained electron dense inclusions presumably calcium which otherwise were absent in the routinely fixed normal mitochondria. In summary, the structural integrity of cell membrane appears to be of vital significance for the cell survival. BIBLIOGRAPHIC REFERENCES: Ashraf, M., F. White, and C.M. Bloor. Ultrastructural alterations in the ischemic myocardium upon reperfusion with calcium-free blood. Fed. Proc. 36:1072 (1977). Ashraf, M., L. Livingston, and C.M. Bloor. Ultrastructural alterations in myocardial vessesl after coronary artery occlusion. In "Scanning Electron Microscopy/1977" (Johari, O., ed.) IIT Research Institute, Chicago (in press) (1977).